Frequency modulation system



Jun. 18, A, IR 2,339,608

FREQUENCY MODULATION SYSTEM Filed Aug. 17, 1942 Invent o 1": Ah on so Alvira,

y Pigg ttorney.

Patented Jan. 18, 1944 2,389,608 FREQUENCY MODULATION SYSTEM Alfonso Alvl a, Schenectady,

General Electric Company,

New York N. Y., assignor to a corporation of Application August 17, 1942, Serial No. 455,016

6 Claims.

My invention relates to frequency modulated transmitting equipment in which the average frequency of the equipment can be varied over a substantial range, and more particularly, to a system of frequency modulation for such equipment.

An object of my invention is to provide a method of frequency modulation in which the modulating effect is heldsubstantially constant over the entire frequency range of the transmitting equipment.

In frequency modulated transmitting equipment, it is a well known practice to vary the frequency of a master oscillator circuit in accordance with the amplitude of modulating signals by means of a reactance circuit. This reactance circuit may comprise an electron discharge device with associated circuits which, in efiect, acts like a variable reactance across the master oscillator tuned circuit, thus altering the frequency at a desired rate; The magnitude of the frequency variation is dependent upon the ratio of the effective reactance of the reactance circuit to that of the master oscillator tuned circuit elements.

In frequency modulated transmitting equipment, where the average frequency of the equipment is variable over a substantial range of frequencies, it is undesirable to have the modulating eiiect of the reactance circuit vary widely with adjustment of the frequency of the master oscillator circuit. It is a further object of my inven tlon therefore, to provide a modulating system for'frequency modulated transmitting equipment which permits the effect of a reactance circuit to be held substantially constant at all adjustments of the frequency of the master oscillator tuned circuit and to maintain a substantially constant amount of modulation over the entire frequency range of the master oscillator.

, The features of my invention which I believe to be novel are set forth with particularity in the appended claims My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 shows the modulation circuit of a frequency modulated transmitter embodying certain features of my invention and Fig. 2 is a modification of the circuit of Fig. invention.

In Fig. 1 the electron discharge device ll) acts as a source of high frequency oscillations, the average frequency of which is controlled by the resonant frequency determining circuit l I. Mod- 1 embodying other features of the ulation of oscillations from the device III is efiected by means of the reactance circuit l2, including the source oi modulating signals 12.

As shown, device ill has an anode ll connected directly to one terminal of the resonant frequency determining circuit II, a control electrode l5 coupled by means of capacitor IE to the other terminal of the resonant frequency determining circuit II and connected by means of resistance I! to ground, and a cathode l8 connected directly to ground. The resonant frequency determining circuit ll comprises an inductance l9 and a variable capacitor 20. A tap on inductance I9 is connected to a source of potential, such as a battery 21, and coupled by means of a condenser 22 to the cathode l8. While I have shown capacitor 20 as a single capacitor, it is of course understood that a plurality of capacitors of the same or of varying size and connected in parallel may be used. In fact, any arrangement may be used which allows the average frequency of the resonant circuit II to be varied over a wide range, for example, from two megacycles to four megacycles.

The reactance circuit 12 comprises an electron discharge device 23 having an anode 24, control electrodes 25 and 2G, and a cathode 21. For a purpose to be explained later, variable capacitor 28 is connected between anode 24 and control electrode 25. Anode 2t, oi the modulating device 28 may be connected to either terminal of the resonant frequency determining circuit II; in the figures of the drawing it is shown as connected to the terminal to which the anode II is connected. Control electrode 25 is connected, by means of resistor 29 and bypass condenser 30 to the cathode of device 23, and by means of resistor 29 and arm 3| to a variable point on resistor 32 connected to ground. In this circuit inductance 83 is inserted to act as a radio frequency choke. Modulating signals, such as from an audio source l3, are connected across resistor 32 and supplied with variable intensity by means of arm 3| to control electrode 25. As shown,

cathode 21 is connected to ground through resistor 3t, resistor 34 being bypassed by capacitor 35. For a purpose to be explained later, capacitors 20 and 28 are shown as arranged for simultaneous operation. Thus, they may be either mounted on the same shaft or otherwise mechanically linked. In order to prevent oscfllations of device 23, control electrode 26, bypassed for alternating currents to cathode 21 by means of capacitor 28, is connected to a source of positive potential in a well known manner to act as the usual screen grid.

2 aeaaeos In the operation of my frequency modulation system, oscillations of an average frequency determined by the capacitance of capacitor 20 in the resonant frequency determining circuit H are generated in the anode circuit of device ill. These oscillations are caused to vary in accordance with the frequency of the modulating signals from the source I! by means of the reactance circuit 12. This circuit, comprising the device 23 and its associated reactance elements, functions as a variable capltance across the tuned circuit l I in accordance with signals from the audio source it. Proper selection of the values of capacitor 28 between the anode and control electrode and the resistor 28 between the control electrode and cathode of the modulating device 23 results in shifting the phase of the modulator anod current so that it is substantially 90' out of phase with the anode current of the oscillator l0. Thus, the modulator functions as a capacitance connected across the frequency determining circuit of the oscillator and modulates the frequency of the oscillations in accordance with the instantaneous intensity of signals, the rate of variation of the frequency of the oscillations corresponding to rate of variation of the instantaneous intensity of the signals, 1. e., the frequency of the signals varying in accordance with the voltage impressed on control electrode 25. Therefore, when an audio frequency signal is applied to control electrode 25 of modulator 23, the oscillator is subjected to the efiect of a varying capitance across the frequency determining circuit ii, causing the oscillator frequencies to swing above and below the mean carrier frequency at the rate determined by the applied audio frequency. The magnitude of frequency variation is dependent upon th ratio of the effective capacitance of the reactance circuit I! to that of the master oscillator tuned circuit.

In order that the effect of the reactance circult remain constant-for different values of the capacity of variable master oscillator capacitor 20, capacitor 28 in the reactance circuit i mechanicaily coupled with capacitor ill for simultaneous variation of capacity therewith. In this manner the ratio of the effective capacitance of the reactance circuit to that of the oscillator tuned circuit may be maintained virtually constant; hence the magnitude of frequency variations of the oscillations, i. e., the amount of modulation, is maintained substantially constant. By properly proportioning capacitor 28 and resistor 29 with respect to capacitor 20, the reactance circuit I! may be designed for all values of capacity of the capacitor 20 and for a predetermined intensity of audio signal to produce a constant modulation of the oscillations in th anode circuit of the device Ill.

In the portion of the transmitter circuit shown in Fig. 2, control electrode 26 of device It is supplied with a unidirectional potential by the variable contact arm 31 on potentiometer 38 connected across battery 2|. In this modification, the effective value of resistor 29 in the reactance circuit II is determined by the position of the variable contact arm 89. As shown on the drawins, capacitors 20 and 28 and contact arms ll, 31, and 3B are mechanically linked together for uncontrolled operation. As explained later. the

number of these elements so unicontrolled may be chosen to meet givenoperating conditions.

As previously explained, variation of the value of either capacitor 28 or resistor 28 changes the apparent reactance of the reactance circuit i2. Hence, the amount of modulation may be controlled by variation of either of these parameters. Adjustment of the contact arm 3| affects the intensity of the signals supplied to the control electrode of modulator 28 and changes the modulating effect of the device 23. Adjustment of this parameter then provides a third means for controlling the amount of modulation as the average frequency of oscillations is adjusted by variation of capacitor 20 in the resonant frequency determining circuit. Variation of a fourth parameter. the potential applied to the control electrode 28, alters the trans-conductance of the device 23 and its effect on the reactance circuit. In practical application any, or all, of these variables may be linked with the capacitor 20. The particular ones to be so unicontrolled depends upon both the range of the average frequency of the oscillations of the device HI and the amount of modulation desired.

It will thus be seen that I have provided a frequency modulating system which permits the effect of the reactance circuit if to be held approximately constant for all adjustments of the variable capacitor 20 in the resonant frequency determining circuit of the master oscillator, thus keeping the modulation effect substantially constant over the entire frequency range of the master oscillator.

While I have shown particular embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desir to secure by Letters Patent of the United States, is:

1. In a radio transmitter the combination of, a source of oscillations, means for modulating a characteristic of said oscillations, means for controlling the amount of said modulation, means for varying the average frequency of said oscillations, said controlling means being mechanically connected with said frequency varying means for simultaneous adjustment therewith to maintain the amount of said modulation substantially constant as said average frequency is varied over a substantial range.

2. I n a radio transmitter the combination of, a source of oscillations including a resonant frequency determining circuit, means for modulating the frequency of said oscillations comprising a reactance circuit connected in shunt with said resonant circuit, said reactance circuit including a variable impedance, adjustable means for varying the frequency of said resonant circuit, and means interconnecting said frequency varying means and said impedance for simultaneously varying the impedance of said reactance circuit as said frequency varying means is adjusted thereby to maintain the amount of modulation substantially constant as said frequency varies over a substantial range.

3. In combination, a source of oscillations including a resonant frequency determining circuit, means for varying the average frequency of said oscillations, means for frequency-modulating said oscillations comprising a reactance circuit connected with said frequency determining circuit, means for varying the apparent reactance of said reactance circuit presented to said frequency determining circuit, and means interconnecting said frequency varying means and said reactance yam ing means for maintaining the amount of frequency modulation substantially constant as said a,sso,eoa 3 average frequency is varied over a substantial means is varied, said ircquency varying means range. v and said conductance adiusting means being so 4. A frequency modulated transmitting sy tem proportioned that the modulating efieet oi said comprising a source or oscillations including a modulating means is maintained substantially resonant irequency determining circuit, means 5 constant as said average frequency is varied over [or varying the average irequency 01 said osciia substantial range.

lations, means for frequency-modulating said 6. A frequency modulated transmitting s stem oscillations comprising an electron discharge decomprising a source of oscillations including a vice having an anode connected with said resresonant frequency determining circuit, means onant circuit, a cathode, and a control electrode, in or varying the average frequency of said oscilsaid control electrode and said cathode, a varoillations comprising an electron discharge device iable reactance connected between said control having an anode connected with said resonant electrode and said anode, and means 101' simultacircuit, a cathode, and a control electrode, a varneously varying said reactance as said frequency iable reacts-nee connected between said control varying means is varied, whereby the amount of electrode and said anode, means for connecting frequency modulation caused by said modulating modulating signals between said control electrode means is maintained substantially constant as and said cathode, means for varying the intensaid average frequency is varied over a substansity of said signals, and means for simultaneously tial range. in adiusting said intens ty varying means as said 5. In combinatioma source of oscillations inirequency varying means is adjusted, said inteneluding a resonant frequency determining circuit, sity varying means and said irequency v rying means for varying the average frequency or said means being so proportioned that the amount of oscillations, means for modulating said oscillafrequency modulation caused by said modulating tions comprising an electron discharge device 5 eans is maintainedsubstantially constant as connected in shunt with said frequency detersaid average frequency is varied over a substanmining circuit, and means linked with said tretial range. quency varying means for adjusting the conduc- ALF NSO ALVIRA- tance of said device as said frequency varying 

